Turf grass (sod) is a living organism that must be handled properly to ensure its survival when it is removed from one location and transplanted to another. Sod is generally harvested using large machinery that cuts slabs of sod from the soil and stacks them on pallets. The speed at which a machine can stack slabs often determines the speed at which the machine can harvest sod.
FIG. 1 illustrates a portion of a sod harvesting machine 100 that includes a typical cutting head, conveyor system, and stacking system. The cutting head of FIG. 1 includes a first blade 111, a second blade 112, and a roller 113. Blade 111 is periodically lowered into the sod to make vertical cuts defining an initial width of the slabs. Blade 112 oscillates back and forth underneath the sod to sever the slab from the underlying soil. Roller 113 applies pressure to the sod as it is cut to facilitate the cutting of clean slabs.
Slabs cut by the cutting head are routed to conveyor 110a which lifts the slabs up to conveyor 110b. Before the slabs reach conveyor 110b, they are cut in half by a third blade 114. For example, a slab is cut into two slabs 102a, 102b of equal width. The two slabs 102a, 102b then pass on to conveyor 110b which positions the slabs underneath stacking head 120a. Stacking head 120a descends to the slabs, picks them up (e.g. via hooks), moves overtop a pallet, and drops the slabs on the pallet. This process continues until a pallet is filled.
To provide adequate spacing between each slab that is cut by blade 112 (as well as to facilitate cutting of slabs by blade 112), conveyor 110a is rotated at a rate that is faster than the speed at which the harvesting machine (e.g. a tractor) travels. In this way, a gap is created between each piece on conveyor 110a thus providing the necessary time for stacking head 120a to travel back and forth between conveyor 110b and the pallet.
This gap, however, necessitates the use of blade 114 because otherwise, the spacing between two slabs would be too great for stacking head 120a to pick up multiple slabs. To address this, the slabs are initially cut to have a width double that of the intended size. The wider slabs are then cut in two by blade 114 to create two side by side slabs. These side by side slabs are then picked up and stacked. Accordingly, the rate at which stacking head 120a can pick up slabs limits the rate at which slabs can be cut from the soil.
To address this limit, harvesting machines with multiple stacking heads have been designed. FIG. 2 illustrates an example of a harvesting machine 200 having two stacking heads 120a and 120b. Because harvesting machine 200 has two stacking heads 120a, 120b, there is no need for blade 114. In other words, each stacking head can pick up a single slab, and therefore, the two stacking heads do not need to pick up slabs in unison. Therefore, the use of two stacking heads accounts for the gap that exists between slabs on conveyor 110a. 
Stacking heads 120a, 120b are configured to descend to conveyor 110b at the moment when a slab is underneath the stacking head, to pick up the slab (e.g. via hooks), and to stack the slabs on a pallet. For example, as shown in FIG. 2, stacking head 120a has picked up slab 102a, and stacking head 120b is waiting until slab 102b is positioned directly below the head at which point stacking head 120b will descend to pick up slab 102b and stack it. The use of multiple stacking heads allows sod to be cut at a quicker rate than when a single stacking head is used in conjunction with blade 114. However, using multiple stacking heads is more complex, expensive, and requires more frequent maintenance.